The main objective of this studyis to examine the changes in body composition for obese children and adolescents during a multidisciplinary weight loss program. In the United States obesity is one of the most prevalend and serious nutritional problems affecting children and adolescents. Strategies are needed to positively affect obese children and adolescents because there are significant complications in the areas of psychological development, orthopedic (bones and joints) injury, exercise tolerance, high blood pressure, elevated blood cholesterol and lipoproteins, and abnormal glucose metabolism. The main therapeutic goal in treating obese children and adolescents is the selective loss of total body fat mass without a detrimental change in total body fat-free mass or linear growth. Measurements of body composition are useful in assessing obesity and in monitoring the effects of intervention programs. To follow the changes in body composition (body fat, soft-lean tissue and bone) throughout the weight loss program, the obese children and adolescents will undergo anthropometric assessments, dual-energy X-ray absorptiometry, and abdominal computer tomography scan at baseline, 10 weeks and at 1-year. In addition, they will answer a series of psychosocial questionnaires, and daily calorie diaries and activity logs.